1. Field of the Invention
This invention relates generally to electromagnetic radiation detectors and more specifically to photosensitive silicon devices.
2. Description of Related Art
Charge-coupled devices (CCDs) were first used as computer memory devices. Their utility as imaging detectors was developed in the 70s. James Janesick explains the general operating principle of CCDs as imaging devices in his article, "CCDs: The Inside Story", CCD Astronomy, Winter 1997. In that article, he also describes the main current problems with CCD performance. Among those problems are absorption in the ultraviolet region and quantum efficiency (QE).
A number of techniques have been employed to enhance blue and ultraviolet sensitivity. One technique is known as `backside illumination`. The `frontside` of the CCD is the side of a silicon wafer on which the electronic circuitry and gates have been fabricated; the `backside` is the other side of the wafer. When a photon enters the silicon wafer (also called the substrate) the photon is absorbed and gives rise to electronic charge (the photoelectric effect). Typically, the electronic circuitry on the frontside is made of polycrystalline silicon (`polysilicon`) instead of metal because polysilicon is less reflective than metal circuitry and thus results in less photon loss when frontside illumination is used. For backside illumination, the image is focused on the backside and electrons generated by the photoelectric process must traverse the interior thickness of the silicon substrate to the silicon region just under the electronic circuitry on the frontside. Because the frontside circuitry is bypassed in backside illumination this method results in a high QE. Unfortunately, however, electrons do not traverse the thickness of the silicon substrate efficiently unless it is extremely thin (about 10 microns thick). Thinning the substrate to this extent is difficult and expensive, and has a low manufacturing yield. The thinning process is described by Burke et al., "Soft-X-Ray CCD Imagers for AXAF", IEEE Transactions on Electron Devices, 44(10):1633-42,1997.
Thinning the silicon substrate that provides the photosensitive volume (i.e. the volume in which a photon is absorbed and creates a charge carrier) also makes it difficult to detect long wavelength photons, because the thinned wafer is likely to be transparent to photons in the red and infrared. The long wavelength photon is not absorbed by the thinned photon-sensitive silicon substrate and thus is not detected. That means a single conventional back-illuminated device cannot be used to detect both short and long wavelength photons.
Sensitivity of CCDs is partly a function of the extent to which charge in individual pixels can be stored without current leakage or extraneous current entering the pixel well. A technique called pinning is used to limit dark current, and to influence residual image, pixel non-uniformity and well capacity (James Janesick and Tom Elliot, "History and advancement of large area array scientific CCD imager," in Astronomical CCD Observing and Reduction, Astronomical Society of Pacific Conf., vol. 23, BookCrafters, Inc., Tucson, Ariz.; 1992).